The present invention relates to a linear power amplifier and a method for a linear power amplification. The apparatus and the method are particularly relates to a linear amplification suitable for use in a modulated digital radio frequency (RF).
A digital telecommunication system is considered to be much more superior to an analog telecommunication system for producing a better voice quality and higher communication capacity. Typically, a mobile telecommunication system consists of a base station (BS) and a mobile telephone (MT) for linking subscribers"" voice signals with the base station. At the base stations, the mobile telephone signals are transmitted to a mobile telephone via a mobile switching center (MSC). The base station uses a plurality of power amplifiers to generate a radio frequency (RF) signal at a specific level.
In order to use the limited frequencies, effectively, the digital telecommunication system uses a variable envelope modulation technology such as quadrature amplitude modulation (QAM) or quadrature phase shift keying (QPSK). Before being up-converted by a carrierband, the baseband signals pass through a pulse-shaping filter causing the baseband signals to be narrower frequency band. Then, these variable envelope signal waves pass through a non-linear power amplifier (NLPA) for amplification. However, the power amplifier employed in the communication system inherently distort its input signals as they are amplified. Thus, the amplification becomes a source of channel-to-channel interference in the adjoining channel and the distortions in the amplitude and phases of the output signals in the receiving-end. To combat such problem, numerous linearization techniques are developed to improve the performance of the non-linear power amplifier (NLPA).
Accordingly, few control techniques, e.g., feed-forward control loops and pre-distortion, have been developed to increase the linearity, i.e. reducing the error of amplifier systems. The feed-forward system includes subtracting a proportion of the power amplifier output signal from a proportion of the input signal to obtain an error signal, amplifying the error signal, and then subtracting the amplified error signal from the power amplifier output signal. That is, the feed-forward system amplifies analog RF signal on carrierband linearly by extracting only the distortion component from the power amplifier output signal making the distorted signals out of phase, amplifying the distorted signals, and then cancelling out the distorting components applied by the amplifier.
FIG. 1 represents a schematic diagram for known feed-forward linear amplification system. A carrierband signal inputted to the amplification system is supplied to a gain-phase regulator. The gain-phase regulator is made up of a variable attenuator (110) and a phase shifter (115) for regulating the gain and the phase of the carrierband signal. The output signal of the phase shifter (115) is inputted to a main amplifier (120), and the main amplifier (120) amplifies the inputted signal to a specific level and also generates distorted amplification signals. The distorted amplification signals from the main amplifier (120) are then outputted to a first delay means (130). At the same time, the carrierband input signal is inputted to a second delay means (140) via a divider (105). The second delay means (140) delays the received carrier input signals and transmits the delayed carrierband signals to a count-balancing means (145). The count-balancing means (145) combines the distorted amplification signal outputted from the main amplifier (120) with the carrierband input signal outputted from the second delay means, yielding a reverse distorted signal which has 180xc2x0 phase difference in comparison to the output signal from the amplifier (120). The count-balancing means (145) extracts only the distorted signal component of the distorted signal from the main amplifier (120), and thereafter, the distorted signal component is amplified and then subtracted from the amplification output signals, to cancel out any distorting components applied by the amplifier.
Because the feed-forward system improves the intermodulation signal distortion on the carrierband, another technique known as a pre-distortion is used to compensate distorted component from the output of the power amplifier on a baseband. The pre-distortion technique provides a pre-generated distortion component to the power amplifier. The pre-distortion technique involves demodulating an output signal of a non-linear power amplifier (NLPA), comparing the demodulated signal with an input signal prior to amplification, and then generating a pre-distorted signal to count-balance the magnitude and gain of distortion components of the amplified output signal. The pre-distorted signal is used to pre-distort the input signal on the baseband to yield undistorted amplified replica of the input signal of the power amplifier.
FIG. 2 represents a schematic diagram illustrating known pre-distortion technique. An input signal is transmitted to a power amplifier via a modem (210), a predistorter (220), and a first quad modulator (230). The amplifier generates an amplified output signal and transmits it to a transmission antenna (260). The first quad modulator (230) modulates the input signal using a local oscillating frequency outputted from a local oscillator (270). The amplified output signal from the output of the amplifier (240) passes through a band-pass filter (290) and supplied to a second quad modulator (280) The second quad modulator (280) demodulates the amplified output signal using the local oscillating frequency outputted from the local oscillator (270). The demodulated signal, Vf, is supplied to the predistorter (220). The predistorter compares the Vf and the input signal, Vi, of the amplifier to generate a pre-distorted signal, which compensates the distorted components of the amplified output signal and transmits the pre-distorted signal to the quad modulator (230). The compensation is made in manner in which the amplifier (240) will inherently pre-distort the input signal by amplifying that signal, thereby eliminating the generation of out-of-band components.
As shown in the prior arts, the feed-forward system and the pre-distortion system are developed to linearize the amplified signals on a carrierband and baseband in the analog and digital communication fields. So far, there have been no particular apparatus and method for eliminating the distortion components in the baseband and carrierband of the amplified output signal by incorporating these two techniques to be effectively used in a digital telecommunication system.
U.S. Pat. No. 5,155,448, xe2x80x9cA distortion generator coupled between the input of the amplifier arrangement and the input of the main amplifierxe2x80x9d discloses a method for combining the pre-distortion with the feed-forward technique used on a carrierband only. Thus, there is need to provide a system used in the digital communication system for eliminating the distortion components in the amplified analog signal on carrierband and amplified digital signal on the baseband.
One of the objects of the present invention is to solve the problem of prior art, and to provide an apparatus and a method for linearized power amplification suitable for use in both carrierband and baseband in the digital communication system.
An apparatus and a method for eliminating amplified distortion signal component includes a baseband linearization arrangement for receiving the baseband signal, demodulating a distorted amplifier output signal and comparing the received baseband signal with the demodulated amplifier output signal to generate a pre-distorted signal that can be used to pre-compensate the distorted component applied by a power amplifier, and also includes a carrierband linearization amplification arrangement for amplifying the pre-distorted signal from the baseband linearization arrangement to further linearize the distorted component of the amplifier output signal. The baseband linearization arrangement incorporates the pre-distortion linearization method for modulating the predistorted baseband signal into the carrierband linear-amplification system. The carrierband linear-amplification system amplifies the modulated pre-distorted signal to achieve the first linerization of the distorted component of the amplifier output signal by pre-calculating the predistorted signal to produce a linerized output signal. The carierband linear-amplification system also extracts an error signal from the distorted amplifier output signal, amplifies the error signal to achieve the second linerization by combining the amplified error signal portion to the amplifier output signal.